This invention relates to a process for preparing a catalyst. In particular, the invention relates to a process for preparing a catalyst which is efficient in converting alkanes to unsaturated aldehydes and carboxylic acids, a catalyst prepared from the process, and a process for preparing unsaturated aldehydes and carboxylic acids using the catalyst.
Unsaturated aldehydes and carboxylic acids are important commercial chemicals. Of particular importance is (meth)acrylic acid. The highly reactive double bond and acid function of (meth)acrylic acid makes it especially suitable as a monomer which may be polymerized alone or with other monomers to produce commercially important polymers. These unsaturated acids are also useful as a starting material for esterification to produce commercially important (meth)acrylate esters. Materials derived from (meth)acrylic acid or esters of (meth)acrylic acids are useful as plastic sheets and parts, paints and other coatings, adhesives, caulks, sealants, and detergents as well as other applications.
The production of unsaturated carboxylic acids by oxidation of an olefin is well known in the art. Acrylic acid, for instance, may be commercially manufactured by the gas phase oxidation of propylene. It is also known that unsaturated carboxylic acids may also be prepared by oxidation of alkanes. For instance, acrylic acid may be prepared by the oxidation of propane. Such a process is especially desirable because alkanes generally have a lower cost than olefins. For example, at the time of filing this application propylene costs approximately three times more than propane. A suitable process for the oxidation of alkanes to unsaturated aldehydes or carboxylic acids which is commercially viable has yet to be achieved.
One impediment for the production of a commercially viable process for the catalytic oxidation of an alkane to an unsaturated carboxylic acid is the identification of a catalyst having adequate conversion and suitable selectivity, thereby providing sufficient yield of the unsaturated carboxylic acid end-product. U.S. Pat. No. 5,380,933 discloses a method for preparing a catalyst useful in the gas phase oxidation of an alkane to an unsaturated carboxylic acid. In the disclosed method, a catalyst was prepared by combining ammonium metavanadate, telluric acid and ammonium paramolybdate to obtain a uniform aqueous solution. To this solution was added ammonium niobium oxalate to obtain a slurry. The water was removed from the slurry to obtain a solid catalyst precursor. The solid catalyst precursor was molded into a tablet, sieved to a desired particle size and then calcined at 600xc2x0 C. under a nitrogen stream to obtain the desired catalyst.
The resulting catalyst was asserted to be effective to convert propane to acrylic acid. However, as shown herein, the present inventor was unable to reproduce the asserted results using the preparation method of the ""933 patent. While not wishing to be bound by theory, it is believed that the poor performance of the prior art method of ""933 results from compositional or phase segregation of the component elements of the catalyst, e.g., in the slurry between solid and liquid phases and during calcining between the gas and various solid phases. The present inventor has now discovered a process for preparing a catalyst for catalyzing an alkane into an unsaturated aldehyde or carboxylic acid wherein phase segregation is minimized and improvement in selectivity, conversion, and yield are achieved.
In one aspect of the present invention, there is provided a process for preparing a catalyst including: (A) admixing metal compounds, at least one of which is an oxygen containing compound, and at least one solvent to form a solution; (B) removing the solvent from the solution to obtain a catalyst precursor; and (C) calcining the catalyst precursor at a temperature from 350xc2x0 C. to 850xc2x0 C. under an inert atmosphere to form a catalyst having the formula
AaMmNnXxOo
wherein 0.25 less than a less than 0.98, 0.003 less than m less than 0.5, 0.003 less than n less than 0.5, 0.003 less than x less than 0.5, and o is dependent on the oxidation state of the other elements, and A is selected from Mo, W, Fe, Nb, Ta, Zr, Ru, and mixtures thereof; M is selected from V, Ce, Cr, and mixtures thereof; N is selected from Te, Bi, Sb, Se, and mixtures thereof; and X is selected from Nb, Ta, W, Ti, Al, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pd, Pt, Sb, Bi, B, In, Ce, and mixtures thereof.
In a second aspect of the present invention, there is provided a process for preparing a catalyst including: (A) admixing metal compounds, at least one of which is an oxygen containing compound, and water to form an aqueous solution; (B) removing the water from the aqueous solution to obtain a catalyst precursor; and (C) calcining the catalyst precursor at a temperature from 400xc2x0 C. to 800xc2x0 C. under an inert atmosphere, wherein the inert atmosphere is not flowing over the catalyst precursor, to form a catalyst having the formula
AaMmNnXxOo
wherein 0.35 less than a less than 0.87, 0.045 less than m less than 0.37, 0.020 less than n less than 0.27, 0.005 less than x less than 0.35, and o is dependent on the oxidation state of the other elements, and A is selected from Mo, W, and mixtures thereof; M is selected from V, Ce, Cr, and mixtures thereof; N is selected from Te, Bi, Sb, and mixtures thereof; and X is selected from Nb, Ta, Zr, and mixtures thereof.
In a third aspect, the present invention provides a catalyst including a compound of the formula:
AaMmNnXxOo
wherein 0.25 less than a less than 0.98, 0.003 less than m less than 0.5, 0.003 less than n less than 0.5, 0.003 less than x less than 0.5, and o is dependent on the oxidation state of the other elements, and A is selected from Mo, W, Fe, Nb, Ta, Zr, Ru, and mixtures thereof; M is selected from V, Ce, Cr, and mixtures thereof; N is selected from Te, Bi, Sb, Se, and mixtures thereof; and X is selected from Nb, Ta, W, Ti, Al, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pd, Pt, Sb, Bi, B, In, Ce, and mixtures thereof; wherein said catalyst has a surface area of from 2 to 10 m2/g as determined by the BET method.
In additional aspects of the present invention, there is provided a catalyst prepared by the processes for preparing a catalyst of the present invention and processes for preparing unsaturated aldehydes or carboxylic acids including subjecting an alkane to catalytic oxidation in the presence of a catalyst prepared according to the process for preparing a catalyst of the present invention.